Metal fiber bundles can be obtained in various ways. Metal fibers can be obtained by a method of bundled drawing as described e.g. U.S. Pat. No. 3,379,000.Metal fibers can also be obtained e.g. by drawing till final diameter, also called end drawing. Typically, metal fibers are less than 60 μm in equivalent diameter. A metal fiber bundle is generally characterised as an array of parallel metal fibers. One type of metal fiber bundles include continuous metal fibers e.g. as obtained by bundled drawing or end drawing and combining these metal fibers into a bundle. Such metal fiber bundles can then be combined to produce metal fiber yarns. These yarns have properties such as a determined strength and electrical resistance.
To increase the strength of a metal fiber yarn with continuous metal fibers of a certain thickness, more metal fibers need to be in the yarn. This can be done in two ways: by increasing the amount of metal fibers in the bundles or by increasing the amount of metal fiber bundles in the yarn.
Increasing the amount of metal fibers per bundle in the yarn has, however, a negative effect on the flexibility of the metal fiber yarn. US2003/0006226 describes a heating wire which comprises a yarn comprising metal fibers, wherein the problem of flexibility and break of the yarn is solved by spirally winding the heat resistance wire around the outer circumference of a core wire formed of heat resistant polyamide fibers. However, this spirally winding around the outer circumference of a polyamide fiber core is prone to sleeving.
Using more metal fiber bundles in the yarn has proven to be limited, i.e. an increase in the amount of metal fiber bundles, did not result in the expected and desired increase of the strength of the metal fiber yarn.
It was further noted that an increase in the amount of metal fiber bundles in the yarn also increased the occurrence of sleeving or decomposition of the yarn resulting in bad processability of the yarn, especially when the metal fiber yarns are made through bundled drawing followed by yarn construction on composite level. When such sleeving sensitive metal fiber yarn is used during subsequent processing, congestion in guiding parts or on small passages may occur.
The smaller than expected increase in breaking force of the yarns consisting out of 5 or more continuous metal fiber bundles occurring together with an increase in the sleeving phenomenon, made people in the art conclude that using 5 or more metal fiber bundles in a yarn was not favourable.
Accordingly, this invention seeks to provide metal fiber yarns with higher breaking force without loosing flexibility and without leading to sleeving of the metal fiber yarns.